This invention relates to AC electroluminescent (EL) devices fabricated using thin film and/or thick film technologies. The invention also relates to full colour EL devices.
U.S. Pat. No. 5,432,015, issued Jul. 11, 1995, to Wu et al., and U.S. Pat. No. 5,756,147, issued May 26, 1998, to Wu et al. disclose an electroluminescent laminate structure which combines a thick film dielectric layer with thin film layers, and a rear to front method of forming same on a rigid, rear substrate. Solid state displays (SSD) using this hybrid thick film/thin film technology have been demonstrated to have good performance and brightness (luminosity) in monochrome (ZnS:Mn phosphor) and full colour (ZnS:Mn/SrS:Ce bilayer phosphor) applications (Bailey et al., SID 95 Digest, 1995), however, improvements are still needed.
The potential for EL as a competitive alternative for fabricating flat panel displays has been hindered by the inability to generate bright, stable full colour. This has resulted in EL only penetrating markets for niche applications, in which the inherent benefits of the technology, such as ruggedness, wide viewing angle, temperature insensitivity, and fast time response, are needed.
Two basic alternatives have been used to produce full colour EL devices. One approach is to use patterned phosphors, that is alternating red, green and blue (RGB) phosphor elements in a layer (see for example U.S. Pat. No. 4,977,350, issued Dec. 11, 1990, to Tanaka et al.). This approach has the disadvantage of requiring the three phosphors to be patterned into red, green and blue sub-pixels that make up each pixel, in separate steps. Furthermore, the three colours cannot all be produced brightly enough by currently available EL phosphors to gain the brightness advantage desired. A second approach is to use a colour by white technique, first described by Tanaka et al., (SID 88 Digest, p 293, 1988, see also, U.S. Pat. No. 4,727,003, issued Feb. 23, 1988 to Ohseto et al.). In the colour by white method, the phosphor layer comprises layers of phosphors, typically ZnS:Mn and SrS:Ce, which when superimposed produce white light. Red, green and blue sub-pixels are then obtained by placing a patterned filter in front of the white light. The white phosphor errits light at wavelengths over the entire visible portion of the electromagnetic spectrum, and the filters transmit a narrowed range of wavelengths corresponding to the colours for each sub-pixel. This approach has the disadvantage of relatively poor energy efficiency, in high measure because a high fraction of the light is absorbed in the filters and the overall energy efficiency of the display is correspondingly reduced.
Another requirement for full colour displays is gray scale capability, that is the ability to generate a number of defined and consistent luminosities (light emission intensities) for each sub-pixel. Typically, 256 gray scale luminosities span a range from zero to full luminosity controlled by predetermined input electrical signals for each sub-pixel. This number of gray levels provides a total of about 16 million individual colours.
Electroluminescent displays have pixels and sub-pixels that are defined by intersecting sets of conductor stripes at right angles to one another on opposite sides of a phosphor layer. These sets of stripes are respectively referred to as xe2x80x9crowsxe2x80x9d and xe2x80x9ccolumnsxe2x80x9d. The sub-pixels are independently illuminated using an addressing scheme called passive matrix addressing. This entails sequentially addressing the rows by applying a short flat-topped electrical pulse with a peak voltage called the threshold voltage sequentially on each of the rows such that the duration of the pulse is less than the time allocated for addressing each row. Electrical pulses, each with a defined and independent peak voltage, termed the xe2x80x9cmodulation voltagexe2x80x9d, are simultaneously applied to each of the columns intersecting the addressed row. This provides independently controllable voltages across the sub-pixels making up the pixels along that row, in accordance with the instantaneous luminosity required for each sub-pixel to achieve the desired pixel colours. While each row is being addressed, the remaining rows are disconnected, or are connected to a voltage level near zero. Independent operation of all sub-pixels on the display requires that sub-pixels not on the addressed row do not illuminate. The electro-optical characteristics of the sub-pixels on an electroluminescent display facilitate meeting this requirement, by virtue of the fact that no luminosity is generated if the voltage across the sub-pixels is below the threshold voltage.
The time required to address all the rows in a display is called a frame, and for video images, the frame repetition rate must be at least about 50 Hz in order to avoid image flicker. At the same time there is a maximum frame repetition rate, typically about 200 Hz, that is achievable due to a limitation on the voltage rise time associated with the electrical characteristics of the display and its associated electronics. In principle, a measure of gray scale can be achieved by controlling the average pixel luminosity by modulating the average frame rate. This requires omitting a fraction of the electrical pulses over a suitably short period of time. In practice, however, due to the limited range of frame rates, only a few levels of gray scale can be realized this way. Another option, called dithering, is to extinguish one or more pixels in the immediate vicinity of a pixel where reduced luminosity is required, thereby spatially modulating luminosity. This technique, however, causes a loss of display resolution and image quality.
The preferred method of gray scale control is to control the instantaneous sub-pixel luminosity, which must be done by modulating the electrical pulse peak voltage, pulse duration or pulse shape. At the same time, to minimize power consumption in electroluminescent displays addressed using passive matrix addressing, it is desirable to have the row voltage as close as possible to the threshold voltage above which luminosity is generated. This requires the threshold voltage for all sub-pixels to be equal.
Filters used to tailor the spectral emission characteristics of sub-pixels typically do not have ideal characteristics. They do not have perfect transmission in the desired wavelength ranges to achieve the desired red, green and blue colours, and they have some optical transparency in the wavelength ranges where they should be opaque. These deviations from ideal behavior impose design limitations on the overall pixel design. For example, the polymer based blue filters commonly used for electroluminescent and other types of flat panel displays have some transmission also in the red portion of the spectrum. The need to suppress red contamination of the blue pixel requires that thicker polymer films be used, which reduces the transparency in the desired blue wavelength range. They also have some transparency in the green wavelength range introducing a similar requirement for thicker polymers that are less transparent to blue light. To meet the requirements for full colour displays, the ratios of luminosity for red:green:blue sub-pixels should be 3:6:1, to give a white colour for that pixel. The CIE colour coordinates for red sub-pixels should be in the range 0.60 less than x less than 0.65 and 0.34 less than y less than 0.36. The CIE colour coordinates for green sub-pixels should be in the range 0.35 less than x less than 0.38 and 0.55 less than y less than 0.62. For blue sub-pixels the CIE colour coordinates should be in the range 0.13 less than x less than 0.15 and 0.14 less than y less than 0.18. The combined (white) luminosity for a pixel comprising red, green and blue sub-pixels should be at least about 70 candelas per square meter (cd/m2) and the CIE colour coordinates for full white should be in the range 0.35 less than x less than 0.40 and 0.35 less than y less than 0.40. Higher luminosity is desirable for some applications.
Phosphors useful in electroluminescent displays are well known, and consist of a host material and an activator or dopant. The host material is usually a compound of a Group II element of the periodic table, with a Group VI element, or is a thiogallate compound. Examples of typical phosphors include zinc sulfide or strontium sulfide, with a dopant or activator which functions as the luminescent center when an electric field is applied across the phosphor. Typical activators with phosphors based on zinc sulfide include manganese (Mn) for an amber emission, terbium (Tb) for a green emission and samarium (Sm) for a red emission. A typical activator with phosphors based on strontium sulfide is Ce for a blue-green emission. It is conventional to refer to phosphors as, for example, SrS:Ce to designate a phosphor based on SrS doped with Ce, and ZnS:Mn to designate a phosphor based on ZnS doped with Mn, and this convention is used herein. It is also conventional, when using the formula for the phosphor, for example as in ZnS, to mean phosphors which are formed predominantly from a stoichiometric zinc sulfide. Other elements might be included in the host material for the phosphor, however it is typically still referred to as a phosphor based on the predominant component of the host material. Thus for instance when referring to a phosphor based on zinc sulfide, or a zinc sulfide phosphor, the terminology includes both pure zinc sulfide as a host material and, for example, the phosphor Zn1xe2x88x92xMgxS:Mn (designating a phosphor based on zinc sulfide but also including magnesium sulfide in the zinc sulfide host material, doped with Mn), although it is also understood that ZnS and ZN1xe2x88x92xMgxS are different host materials. This phosphor terminology is used herein and the patent claims.
The present invention provides improvements in a thick film dielectric layer for use in a hybrid thick film/thin fiim electroluminescent device. The thick film dielectric layer of this invention is formed by thick film techniques from a dielectric material having a high dielectric constant, generally greater than about 500. The improvements are realized by compressing, for example by isostatic pressing, the thick film dielectric layer prior to sintering, to significantly reduce the porosity and the thickness of the layer, and to significantly increase the dielectric strength of the layer. The result is an unexpected improvement in the dielectric properties of the dielectric layer, significant reductions in the thickness, porosity, void space and interconnectedness of the void space of the layer, and an improvement in the surface smoothness of the layer, leading to more uniform luminance and reduced dielectric breakdown in electroluminescent displays formed therefrom.
Electroluminescent laminates made with the thick film dielectric as set forth in U.S. Pat. No. 5,432,015, generally show uniform luminosity as viewed by the naked eye, but when viewed under a xc3x97100 microscope show a mottled appearance with some areas brightly illuminated and other areas dimly illuminated or not illuminated at all. When the driving voltage is near the threshold voltage this mottled appearance is most pronounced. The effect is diminished as the voltage is increased above this value and all regions become illuminated. The effect of this behavior is that the onset of luminosity occurs gradually as the voltage is raised above the nominal threshold value and the rate of increase in the average luminosity with increasing voltage is relatively low. The scale of the observed variability of the luminosity is of the order of 10 xcexcm. In contrast, electroluminescent laminates made with a thick film dielectric layer which has been isostatically pressed prior to sintering, in accordance with this invention, do not show this mottled characteristic of the luminosity near the threshold voltage and increases nearly linearly up to about 50 volts above the threshold voltage, so that the average luminosity at a fixed voltage above the threshold voltage is about 50% higher than for an otherwise identical electroluminescent laminate. xe2x80x9cUniform luminosityxe2x80x9d, as used herein, means the luminosity resolved to a scale of about 10 xcexcm appears uniform.
Broadly stated, in one aspect of the invention there is provided a method of forming a thick film dielectric layer in an EL laminate of the type including one or more phosphor layers sandwiched between a front and a rear electrode, the phosphor layer being separated from the rear electrode by the thick film dielectric layer, comprising:
depositing a ceramic material in one or more layers on a rigid substrate providing the rear electrode, by a thick film technique, to form a dielectric layer having a thickness of 10 to 300 xcexcm;
pressing the dielectric layer to form a densified layer with reduced porosity and surface roughness; and
sintering the dielectric layer to form a pressed, sintered dielectric layer which, in an EL laminate, has an improved uniform luminosity over an unpressed, sintered dielectric layer or the same composition.
In another broad aspect, the invention provides an improved combined substrate and dielectric layer component for use in an EL laminate, comprising:
a rigid substrate providing a rear electrode;
a thick film dielectric layer on the substrate providing the rear electrode, the thick film dielectric layer being formed from a pressed, sintered ceramic material having, compared to an unpressed, sintered dielectric layer of the same composition, improved dielectric strength, reduced porosity and uniform luminosity in an EL laminate.
In still a further broad aspect, the invention provides an EL laminate, comprising:
a planar phosphor layer;
a front and rear planar electrode on either side of the phosphor layer;
a rear substrate providing the rear electrode, the rear substrate having sufficient mechanical strength and rigidity to support the laminate; and
a thick film dielectric layer on the substrate providing the rear electrode, the thick film dielectric layer being formed from a pressed, sintered ceramic material having, compared to an unpressed, sintered dielectric layer of the same composition, improved dielectric strength, reduced porosity and uniform luminosity in an EL laminate.
The present invention further provides a patterned phosphor structure particularly useful in AC thin film/thick film electroluminescent devices, and also useful in AC thin film electroluminescent devices if the thickness of the phosphor over the sub-pixels is not too great. In the phosphor structure of the invention, the emitted light from the phosphor underlying the red, green and blue sub-pixels falls within a narrowed wavelength range of the visible electromagnetic spectrum that more closely matches the range transmitted by the respective filters. In this manner, both the luminosity and the energy efficiency of the display can be substantially increased over the values achievable with a conventional colour by white phosphor design. Another feature of the patterned phosphor structure of the present invention is that the sub-pixel threshold voltages can be made equal and, the relative luminosities of the sub-pixels can be set so that they bear set ratios to one another at each operating modulation voltage used to generate the desired luminosities for red, green and blue. Preferably, the set ratios remain substantially constant over the full range of the modulation voltage, for proper colour balance. Most preferably, for a full colour display, the set luminosity ratios for the red, green and blue sub-pixels are in the ratio of about 3:6:1, or sufficiently close to this ratio so as to enable adequate colour fidelity (gray scale).
To reduce the negative impact of the limitations inherent in filter characteristics, it is desirable to use a phosphor for the blue sub-pixels that does not emit significant intensities of green or red light. Cerium doped strontium sulfide (SrS:Ce), optionally codoped with phosphorus, preferably prepared as set out herein, provides desirable CIE colour coordinates and luminosity for the blue, and optionally for the green sub-pixels. For green sub-pixels, manganese doped zinc sulfide (ZnS:Mn) does not generally provide an adequate luminosity when filtered to provide acceptable colour coordinates, but in accordance with this invention, it can be combined with cerium doped strontium sulfide to give higher luminosity with good colour coordinates. Alternatively, Zn1xe2x88x92xMgxS:Mn, which, with an appropriate ratio of Zn to Mg, has a higher luminosity in the green region of the spectrum than does ZnS:Mn, can be used for the green sub-pixels, optionally with ZnS:Mn. Either or both of the Zn1xe2x88x92xMgxS:Mn or the ZnS:Mn phosphors can be used for the red sub-pixels, x being between 0.1 and 0.3.
In accordance with this invention, one or more means are included with the one or more of the phosphor deposits for setting and equalizing the threshold voltages of the sub-pixels, and for setting the relative luminosities of the sub-pixels so that they bear set ratios to one another at each operating modulation voltage used to generate the desired luminosities for red, green and blue. Threshold voltage means the highest amplitude of a voltage pulse that, when applied to a sub-pixel at the desired repetition rate, generates a measurable filtered luminosity less than the lowest specified gray scale. luminosity for that sub-pixel. Thus, the means for setting and equalizing the threshold voltages also functions to set the relative sub-pixel luminosities so that they bear set ratios to one another over the full range of the modulation voltage used. Generally, the means is one or more of (a) a threshold voltage adjustment layer formed from a dielectric or semiconductor material which is located in one or more of the positions of over, under and embedded within one or more of the phosphor deposits, and/or (b) one or more of the phosphor deposits being formed with different thicknesses.
It should be noted that the terms xe2x80x9csub-pixelxe2x80x9d and xe2x80x9csub-pixel phosphor elementsxe2x80x9d are used interchangeably herein to refer to the phosphor deposits for a particular red, green or blue sub-pixel element, along with any threshold voltage adjustment deposit associated with that sub-pixel element.
Appropriate colour filters can be chosen for the three sub-pixels to achieve self-consistent optimization of luminosity and colour coordinates for each, and overall pixel energy efficiency. The present invention has application to other colour phosphors, the strontium sulfide and zinc sulfide phosphors being representative only. Usually, at least two different phosphors are used, each being formed from different host materials. It is also possible to extend the present invention to three or more different phosphor layers for further optimization.
Broadly stated, the invention provides a patterned phosphor structure having red, green and blue sub-pixel phosphor elements for an AC electroluminescent display, comprising:
at least a first and a second phosphor, each emitting light in different ranges of the visible spectrum, but whose combined emission spectra contains red, green and blue light;
said at least first and second phosphors being in a layer, arranged in adjacent, repeating relationship to each other to provide a plurality of repeating at least first and second phosphor deposits; and
one or more means associated with one or more of the at least first and second phosphor deposits, and which together with the at least first and second phosphor deposits, form the red, green and blue sub-pixel phosphor elements, for setting and equalizing the threshold voltages of the red, green and blue sub-pixel phosphor elements, and for setting the relative luminosities of the red, green and blue sub-pixel phosphor elements so that they bear set ratios to one another at each operating modulation voltage used to generate the desired luminosities for red, green and blue.
Suitable materials for the threshold voltage adjustment layers are those which, when deposited as a layer, at an appropriate thickness, will not conduct until the voltage across the patterned phosphor structure exceeds the threshold voltage for an otherwise identical patterned phosphor structure that does not include the threshold voltage adjustment layer. A suitable material can be chosen by examination of its dielectric constant and dielectric breakdown strength to meet the above condition, with materials having relatively high dielectric constants and dielectric breakdown strengths as compared to those of the phosphor materials being preferable. The materials for the threshold voltage adjustment layer are compatible with those materials that are in contact with them in the patterned phosphor structure, and are chosen from dielectric materials and semiconductors. By semiconductors is meant both intrinsic semiconductors, and semiconductors with deep impurity levels that have effective electronic band gaps that are comparable to, or larger than, the effective band gap of the phosphor material. Examples of suitable materials include binary metal oxides such as alumina and tantalum oxide, binary metal sulfides such as zinc sulfide and strontium sulfide, silica, and silicon oxynitride. The suitability of these materials is dependent on the properties of the interface between the materials and any phosphor materials and the dielectric materials in contact with them. In general, when the phosphor deposit is of a phosphor which is based on zinc sulfide, the preferred threshold voltage adjustment material is a binary metal oxide, most preferably alumina.
Alternatively, or in addition, the means for setting and equalizing the threshold voltages and for setting the relative luminosities comprises forming the first and second phosphor deposits with different thicknesses so as to balance the threshold voltages and the luminosities of the sub-pixel elements. In this case, the overall colour balance can be achieved for a pixel by setting the luminosities for the sub-pixel by using different sub-pixel element areas, for instance by making the sub-pixel elements of the less efficient phosphors wider than the width of the sub-pixel elements with the more efficient phosphors.
The patterned phosphor structure of this invention allows for correct CIE colour coordinates for a full colour display to be achieved for all operating modulation voltage levels, while allowing for the equalizing of the threshold voltages of the sub-pixel elements. The means for setting and equalizing the threshold voltages, and for setting the relative luminosities of the red, green and blue sub-pixels may also comprise, in addition to the threshold voltage adjustment deposits and/or altering the thicknesses of the phosphor deposits, varying one or more of the following in order to set the relative luminosities:
i. the areas of the phosphor deposits; and
ii. the concentrations of a dopant or co-dopant in the phosphor deposits.
Preferably, the first and second phosphors are of different host materials, such as a strontium sulfide phosphor or a zinc sulfide phosphor. Generally, a different host material implies that a different element has been introduced to the phosphor host material at an atomic percent greater than about 5 atomic percent. Preferred first and second phosphors are SrS:Ce and ZnS:Mn; SrS:Ce and Zn1xe2x88x92xMgxS:Mn; or SrS:Ce with layers of both ZnS:Mn and Zn1xe2x88x92xMgxS:Mn, it being possible for the SrS:Ce to be codoped with phosphorus. These are examples of zinc sulfide and strontium sulfide phosphors which, if they were superimposed, would have a combined emission spectrum which covers the wavelengths of white light (individual visible spectra for ZnS:Mn and SrS:Ce are shown in FIGS. 7 and 8 respectively). Within the scope of the present invention, each of the first and second phosphor deposits may comprise one or more layers of a same or different phosphor for each sub-pixel element, and each of the phosphor deposits may themselves be composed of one or more phosphor compositions (i.e. mixtures of more than one phosphors). As set out below, the phosphor structure of this invention may be provided on one or more layers. For example, in a single layer phosphor structure, as set forth in Example 3, the phosphors can be arranged such that Zn1xe2x88x92xMgxS:Mn forms the red and green sub-pixel elements, while SrS:Ce forms the blue sub-pixel element. A threshold voltage adjustment layer of a binary metal oxide such as alumina can be provided over the red and green sub-pixel elements to achieve the desired luminous intensity ratios between the sub-pixel elements. Alternatively, as set forth in Example 4, SrS:Ce deposits can be used for the blue sub-pixel elements, and a layer of Zn1xe2x88x92xMgxS:Mn between layers of ZnS:Mn can be used for the red and green sub-pixel elements. The stacked zinc sulfide phosphor deposits of this embodiment can be formed thick enough to equalize the threshold voltages between the sub-pixel elements. To achieve the desired relative luminosities between the sub-pixel elements, the SrS:Ce deposits for the blue sub-pixels can be made wider than the sub-pixels for red and green. Alternatively, as set forth in Example 5, SrS:Ce deposits can be used for both the green and blue sub-pixel elements, and ZnS:Mn can be used for the red sub-pixel elements. A threshold voltage adjustment layer of a binary metal oxide such as alumina can be used over the red sub-pixel deposits to equalize the threshold voltages.
When two layers of phosphors are used, as in Example 2, the phosphors may be arranged such that SrS:Ce is patterned in a first layer with ZnS:Mn or Zn1xe2x88x92xMgxS:Mn, and a second layer of SrS:Ce can be formed over the first layer. In this embodiment, the stacked phosphor deposits of SrS:Ce form the blue sub-pixel elements, while the red and green sub-pixel elements are formed by the stacked zinc sulfide phosphor deposit under the SrS:Ce deposit.
Compared to conventional colour by white techniques in which the white light is provided by coplanar, stacked layers of SrS:Ce and ZnS:Mn, the patterned phosphor structure of the present invention has the advantage of being able to provide a thicker layer of SrS:Ce for the blue sub-pixel element, without having an over- or under- layer of ZnS:Mn. This results in increased blue luminance and, since there is no yellow-orange light being emitted in the blue sub-pixels, the filtered light from the SrS:Ce phosphor is a more saturated blue.
The patterned phosphor structure of this invention has particular application in hybrid thick film/thin film AC electroluminescent devices such as described in U.S. Pat. No. 5,432,015, in which the EL laminate is fabricated on a rigid rear substrate, with a thick film dielectric layer below the phosphor structure. AC thin film electroluminescent devices (TFELs) have the disadvantage of generally requiring its thin layers to be planarized, that is of even thicknesses. Such devices generally preclude the ability to use colour phosphor sub-pixels of differing thicknesses. However, using a thick film dielectric layer in an EL laminate in combination with the patterned phosphor structure of the present invention allows one to use different thicknesses of the individual phosphor sub-pixel deposits, so as to optimize the colour coordinates and luminosity of a particular sub-pixel element, while still setting and equalizing the threshold voltages for the sub-pixel elements.
The present invention also extends to novel methods for fabricating the patterned phosphor structure of the present invention. Broadly stated, the invention provides a method of forming a patterned phosphor structure having red, green and blue sub-pixel elements for an AC electroluminescent display, comprising:
selecting at least a first and a second phosphor, each emitting light in different ranges of the visible spectrum, but whose combined emission spectra contains red, green and blue light;
depositing and patterning said at least first and second phosphors in a layer to form a plurality of repeating at least first and second phosphor deposits arranged in adjacent, repeating relationship to each other; and
providing one or more means associated with one or more of the at least first and second phosphor deposits, and which together with the at least first and second phosphor deposits, form the red, green and blue sub-pixel phosphor elements, for setting and equalizing the threshold voltages of the red, green and blue sub-pixel phosphor elements, and for setting the luminosities of the red, green and blue sub-pixel elements so that they bear set relative luminosities to one another at each operating modulation voltage used to generate the desired luminosities for red, green and blue; and
optionally annealing the patterned phosphor structure so formed.
Preferably the patterning of the at least first and second phosphor is achieved by photolithographic techniques, including the steps of:
a) depositing a layer of the first phosphor which is to form at least one of the red, green and blue sub-pixel elements;
b) removing the first phosphor material in regions which are to define the other of the red, green and blue sub-pixel elements, leaving spaced first phosphor deposits;
c) depositing the second phosphor over the first phosphor deposits and in the regions which are to define the other of the red, green and blue sub-pixel elements; and
d) removing the second phosphor from above the first phosphor deposits, leaving a plurality of repeating first and second phosphor deposits arranged in adjacent, repeating relationship to each other.
Novel photolithographic techniques have been developed which are particularly useful in patterning strontium and zinc sulfide phosphors, but which have application to other phosphor combinations. In its most preferred embodiments, the photolithographic methods of this invention utilizes a negative photoresist, and has the advantage of needing only one photomask to accomplish the patterning of the red, green and blue sub-pixel elements. In accordance with this method, steps b) through d) include, applying a negative resist to the first phosphor; exposing and developing the resist through a photo-mask in the areas that the first phosphor is to define one or more of the red, green and blue sub-pixel elements; removing the first phosphor as in step b), depositing the second phosphor over the first phosphor deposits and in the regions which are to define the other of the red, green and blue sub-pixel elements; and then removing, by lift-off, the second phosphor from above the first phosphor deposits. Typically in this method, the first phosphor is a strontium sulfide phosphor, most preferably SrS:Ce, which forms the blue sub-pixel elements and optionally the green sub-pixel elements, and the second phosphor is a zinc sulfide phosphor, most preferably ZnS:Mn or Zn1xe2x88x92xMgxS:Mn, or both, which forms the red, and optionally the green, sub-pixel elements. In accordance with the method, the means for setting and equalizing the threshold voltages and for setting the luminosities of the sub-pixel elements can include adding a threshold voltage adjustment deposit beneath, within or above one or more of the phosphor deposits and/or forming the phosphor deposits with different thicknesses, as set out above. In addition, the means for setting and equalizing the threshold voltages, and for setting the luminosities, of the sub-pixel elements may include varying one or more of:
i. the areas of the phosphor deposits; and
ii. the concentrations of a dopant or co-dopant in the phosphor deposits.
The invention also provides a novel photolithographic technique which is particularly useful for patterning a phosphor which is subject to hydrolysis, such as alkaline earth metal sulfide or selenide phosphors. Broadly, the invention provides a method of forming a patterned phosphor structure having red, green and blue sub-pixel elements for an AC electroluminescent display, comprising:
a) selecting at least a first and a second phosphor, each emitting light in different ranges of the visible spectrum, but whose combined emission spectra contains red, green and blue light;
b) depositing a layer of the first phosphor which is to form at least one of the red, green or blue sub-pixel elements;
c) applying a photo-resist to the first phosphor, exposing the photo-resist through a photo-mask, developing the photo-resist, and removing the first phosphor in regions that the first phosphor is to define as one or more of the red, green and blue sub-pixel elements, leaving spaced first phosphor deposits, wherein the first phosphor is removed with an etchant solution comprising a mineral acid, or a source of anions of a mineral acid, in a non-aqueous, polar, organic solvent which solubilizes the reaction product of the first phosphor with anions of the mineral acid, and wherein optionally, prior to removing the first phosphor with the etchant solution, the first phosphor layer is immersed in the non-aqueous organic solvent;
d) depositing the second phosphor material over the first phosphor deposits and in regions which are to define the other of the red, green and blue sub-pixel elements; and
e) removing by lift-off, the second phosphor material and the resist from above the first phosphor deposits leaving a plurality of repeating first and second phosphor deposits arranged in adjacent, repeating relationship to each other.
The invention also extends to EL laminates combining, as described above, a rigid rear substrate, a thick film dielectric layer and the patterned phosphor structure, together with front and rear column and row electrodes on either side of the phosphor layer, in which the front and rear column and row electrodes are generally aligned with the phosphor sub-pixel elements, and bandpass colour filter means aligned with the red, green and blue phosphor sub-pixel elements for passing therethrough red, green and blue light emitted from the phosphor sub-pixel elements.
Another aspect of the present invention provides novel and separate selection criteria for barrier diffusion layers and injection layers useful with electroluminescent phosphors, and particularly useful with the patterned phosphor structure and the thick film dielectric of the present invention. Preferably, a diffusion barrier layer is included above the thick film dielectric layer, or if present, above the second ceramic material. The diffusion barrier layer is composed of a metal-containing electrically insulating binary compound which is compatible with any adjacent layers, and which is precisely stoichiometric, preferably varying from its precise stoichiometric composition by less than 0.1 atomic percent, and having a thickness of 100 to 1000 xc3x85. Preferred materials will vary with the particular phosphors and the materials in the dielectric layers, but most preferred materials are alumina, silica and zinc sulfide. Preferably, an injection layer is included above the thick film dielectric layer, or if present, above the second ceramic material or the barrier diffusion layer, to provide a phosphor interface. The injection layer is composed of a binary dielectric or semi-conductor material which is non-stoichiometric in its composition and which has electrons in a preferred range of energy for injection into the phosphor layer. The material is compatible with adjacent layers and is preferably non-stoichiometric by greater than 0.5 atomic percent. Preferred materials vary with the particular phosphor and the materials in the underlying dielectric layers, but preferred materials for providing optimum electron energies are hafnia or yttria. There is a compromise between optimum electron injection and compatibility with adjacent layers. As a result, sometimes a non-stoichiometric compound cannot be used as an injection layer.
Another broad aspect of the invention provides a method of synthesizing strontium sulfide, comprising:
providing a source of high purity strontium carbonate in a dispersed form;
heating the strontium carbonate in a reactor with gradual heating up to a maximum temperature in the range of 800 to 1200xc2x0 C.;
contacting the heated strontium carbonate with a flow of sulfur vapours formed heating elemental sulfur in the reactor to at least 300xc2x0 C. in an inert atmosphere; and
terminating the reaction by stopping the flow of sulfur at a point when sulfur dioxide or carbon dioxide in the reaction gas reaches an amount which correlates with an amount of oxygen in oxygen-containing strontium compounds in the reaction product which is in the range of 1 to 10 atomic percent.
By xe2x80x9cdispersed formxe2x80x9d, in reference to the source of strontium carbonate, as used herein and in the claims, is meant that the strontium carbonate powder particles are exposed to the process conditions substantially uniformly. This can preferably be achieved by using small batches, using volatile, non-contaminating, clean evaporating compounds or solvents which decompose into gaseous products prior to the onset of the reaction, using fluidized beds or tumbler reactors.
The term xe2x80x9cphosphorxe2x80x9d as used herein and in the claims, means a substance which provides electroluminescence when a sufficient electric field is applied across it, and electrons are injected into it.
The term xe2x80x9cwhite lightxe2x80x9d when used herein and in the claims, when referring to the combined emission spectra of two or more phosphors, means that white light is emitted when the phosphors are superimposed in a manner such that the light can be filtered to provide red, green and blue light.
The term xe2x80x9ccompatiblexe2x80x9d when used herein and in the claims, means that the material is chemically stable to that it does not chemically react with adjacent layers.